How long will it take to reach herd immunity to the coronavirus? A group of experts from the Center for Infectious Disease Research and Policy (CIDRAP) has taken a look back at other pandemics over the past 300 years and concluded COVID-19 will probably be around for 18-24 more months.

“I have said for a long time that when you are trying to understand how infectious disease is going to unfold, you should rely on history as well as models,” Lipsitch told CNN. For instance, pandemic infections don’t tend to die down in the summer, like seasonal flu does., he said.

Covid-19 is most comparable to a pandemic strain of influenza, they said.

“Because of a longer incubation period, more asymptomatic spread, and a higher R0, COVID-19 appears to spread more easily than flu,” they wrote in the report. R0 is the average number of other people infected by each patient.

“A higher R0 means more people will need to get infected and become immune before the pandemic can end,” they add. “Based on the most recent flu pandemics, this outbreak will likely last 18 to 24 months.”

The full report goes into more detail about the comparison between the coronavirus and the flu. It argues that while COVID-19 shares similarities with flu pandemics it is actually worse in several regards. It takes longer to incubate which means people are more likely to have it and not know it. It seems to have a higher rate of asymptomatic infections. The report cites research that an average of around 16% of flu cases on average are asymptomatic but for the coronavirus the figure is currently estimated at 25 percent and could actually be higher. Again this means a lot more people who have the virus but don’t know it (and therefore spread it unknowingly). Third, unlike some viruses the coronavirus is infectious for up to two days before people have symptoms. So even people who eventually realize they are sick will have likely spread the virus before they were aware of it. I previously wrote about a study that suggested the peak of infectiousness for the coronavirus is slightly before the first symptoms.

All of these factors make the R0 higher than for other viruses. The report suggests the bottom line is that we may need to get to the point where 60-70% of the population has had the virus before the spread is truly limited. And getting to that point is what they predict will take two years:

Key points from observing the epidemiology of past influenza pandemics that may provide insight into the COVID-19 pandemic include the following. First, the length of the pandemic will likely be 18 to 24 months, as herd immunity gradually develops in the human population. This will take time, since limited serosurveillance data available to date suggest that a relatively small fraction of the population has been infected and infection rates likely vary substantially by geographic area. Given the transmissibility of SARS-CoV-2, 60% to 70% of the population may need to be immune to reach a critical threshold of herd immunity to halt the pandemic (Kwok 2020).

The report actually spells out three different scenarios for how the spread of the virus could go between now and the point we reach herd immunity. Here’s a graphic showing the three possibilities:

They refer to scenario 1 as “repetitive smaller waves” which could require mitigation efforts to be reinstituted as the waves begin to peak at various times around the country. This is similar to the prediction of a Harvard study published earlier this month.

Scenario 2 is the most worrisome of the three because it suggests the second wave this winter could be much worse than the first. The report points to three previous pandemics, including the flu outbreak in 1918 where the second wave was larger than the first.

And scenario 3 is the least worrisome, though it still involves a lot more deaths over the next 18-24 months. In this scenario the initial outbreak is followed by a “slow burn” in which this drags on but does not require further mitigation efforts such as shutting down businesses.

Of course the main point here is that there is probably no scenario where this is over in a couple of months. It will continue to be with us until we reach herd immunity or until we have a vaccine to speed up herd immunity. But even if scenarios 1 or 2 turn out to be more accurate, the size of those peaks will still depend on mitigation efforts. So wearing masks and social distancing could still make things less bad than they might otherwise be.

The authors of the report suggest authorities plan for the worst case (scenario 2) and get ready to resume tough measures if it looks like the fall/winter is going to be worse than what we’ve already seen.

Overall, it’s a pretty depressing view of the near future. I’m still hoping that Oxford or one of the other teams racing to create a vaccine will be successful and that by this winter we’ll be able to create our own immunity.